1. Field of the Invention
This invention relates to a thermal head driving device in which a plurality of heating resistors are arranged and a printing pulse signal is caused to flow to the resistors to thereby cause the resistors to heat, thus effecting the printing.
2. Description of the Prior Art
A thermal head is generally incorporated in the printer of a small electronic desk-top calculator or the like and receives a printing pulse of several milliseconds obtained from the control circuit of the calculator. During application of the pulse, the temperature of heating resistors constituting the thermal head rises to the order of 300.degree. C., whereby printing is effected on thermosensitive recording paper to record the result of the operation of the desk-top calculator.
Accordingly, application of a printing pulse having a pulse width greater than a predetermined pulse width would cause burning or reduced life of the heating resistors and therefore, for example, where the control circuit fails and an abnormal printing pulse signal is generated, a safety circuit for protecting the thermal head against such abnormality will become necessary.
Further, the thermal head is such that the resistors thereof are caused to heat as described above, whereby printing is effected on thermosensitive paper and therefore, the printing density printed on the thermosensitive paper is greatly affected by the ambient temperature surrounding the thermal head. Accordingly, in order to keep constant the printing density obtained on the thermosensitive recording paper in spite of the temperature difference around the thermal head, a temperature correcting circuit becomes necessary which controls the heating temperature of the heating resistors in accordance with the ambient temperature.
Further, since the thermal head comprises a plurality of heating resistors, some degree of irregularity occurs to the resistance values of the heating resistors, similar to ordinary resistance elements, during the manufacture thereof. Accordingly, in order to eliminate the difference in printing density resulting from the difference in resistance value between thermal heads, the resistance values must be made as uniform as possible and for this purpose, a compensation circuit or a grading circuit becomes necessary which grades (classifies the grade or quality) the heating resistors of the thermal head for each of certain ranges of resistance value and varies the printing condition (the printing pulse width or the printing voltage) by each grade.
Thus, the above-mentioned three types of circuits are very important in order to effect highly reliable printing of good quality on thermosensitive paper by the use of a thermal head and thereby record information, whereas the thermal head driving device of the conventional printer has only one of these three types of circuits and could not well cope with various manufacturing conditions or operating conditions.